In 2011, the Federal Communication Commission (FCC) in the United States reinforced its recommended guidance on how transmit signal power should be calculated for multiple-input multiple-output (MIMO) based systems. The interpretation of this ruling is that “uncorrelated” signal combining techniques, such as Cyclic Shift Delay (CSD), could be correlated over a “narrow” bandwidth and should be subject to the same maximum transmit power limitations as unintentional combining methods such as transmit beamforming. Prior to this recommendation, signals transmitted with CSD by MIMO devices have been treated as a “10log(Num_TX_Ant)” power addition, where “NUM_TX_Ant” is the number of transmit antennas. As a result of this regulation, signals transmitted with CSD will need to be treated as a “20log(Num_TX_Ant)” power addition.
In regulatory frequency bands that have strict maximum power guidelines, e.g., Unlicensed National Information Infrastructure-1 and 2 (UNII-1 and UNII-2) frequency bands in the United States, this ruling has the undesired effect of reducing the total radiated power when transmitting with CSD relative to prior products that were not subject to this ruling. The most dramatic effect of this is when signals are transmitted at lower spatial stream rates in the UNII-1 band. Extreme cases of this for a 4-transmitter architecture result in a 6 dB difference.